Varactors is a diode that is specifically designed to be functional in the reverse-bias electrical condition.  This departs from a traditional diode that is designed to function as a current pathway while positively biased.  When regular diodes are reverse-biased, they exhibit a form of capacitance, which is a negative attribute and so is mitigated as much as possible in the manufacturing process.  Varactors also exhibits capacitance the same way, but it is desirable for the application in which they are used.  The manufacturing process of varactors attempts to increase the capacitance as much as possible.

For varactors, the voltage across its terminals determines what the capacitive value is; there is no constant capacitance rating.  This feature is what makes varactors optimal for some important uses.  If a high capacitance state is desired (with little to no voltage drop) then this acts as low impedance to AC signals.  If a low capacitance state is desired (with a higher reverse-bias DC voltage) then the same signals are filtered out to some extent.  This makes them very popular in tuner circuits and electronic switching, in which case a DC power source can act as a variable capacitance controller for the diode.